1. Field of the Invention
The present invention relates to a lighting control apparatus for a photographing apparatus, and in particular to improvement of the lighting action in the continuous or video mode.
2. Description of the Related Art
A lighting device which uses LEDs is proposed, in place of a lighting device which uses strobe radiation such as a xenon pipe which has been widely used with photographing apparatus, such as a camera. LEDs can be driven at a low voltage, and the circuit construction of the LED is simple.
However, the temperature of an LED goes up due to continuous radiation of the LED. The rise in the LED temperature causes a decrease of the quantity of light emitted by the LED. FIG. 1 shows the relation between the temperature rise and the quantity of light emitted by the LED, so that FIG. 1 shows that the quantity of light, in other words, the brightness of the LED, falls along with the temperature rise of the LED.
Japanese unexamined patent publication (KOKAl) No. 2003-101836 discloses a lighting device for a photographing apparatus. The radiation of light from the LEDs is driven by rectangular pulse signals, and a continuous imaging operation can be carried out while the LEDs radiate light.
Because a turn-off period is provided in the pulse signal, the LED radiation caused by this pulse signal is not continuous, hence the temperature rise of the LED due to the heat that occurs with continuous radiation is reduced in comparison to the temperature produced by LED radiation when the direct current signal is used to drive the LED.
FIG. 2 shows a graph where the horizontal axis represents time and the vertical axis represents temperature. The graph shows the difference in temperature rise of the LED due to the direct current signal ((1) in FIG. 2) and the temperature rise of the LED due to the pulse signal ((2) in FIG. 2).
When the pulse signal and the direct current signal are in the on state, the LED for lighting is illuminated.
In the case of the direct current signal, because a constant current flows during the period for which the LED should be illuminated (T0˜T5), the temperature of the LED continues to go up.
In the case of the pulse signal, the period for which the LED is not illuminated is the period which the signal is in the off state (T1˜T2, T3˜T4), so that the temperature of the LED descends in this period (T1˜T2, T3˜T4). Accordingly, during the illumination period of the LED, the temperature of the LED repeats the rise and descent. As a result, the accumulation of heat is small in comparison with in the case where the direct current signal is used.
However, the above-discussed Japanese unexamined patent publication does not disclose the temperature rise of the LED due to the continuous radiation of the LED in the continuous shot mode.
In this case, a method, where by the heat, which is accumulated in the LED in the previous exposure time, is cooled by turning off the pulse signal during the post-exposure time (the time between the previous exposure time termination and the current exposure time start) is considered. However, when the length of the post-exposure time is shortened in order to increase the number of frames which can be imaged in a unit of time, the cooling of the LED can not be carried out sufficiently by utilizing the post-exposure time. Accordingly, whenever the exposure operation is continuously carried out, the heat of the LED which can not be cooled is accumulated, so that the temperature of the LED goes up.